PRODUCTS OF RESPIRATION OF FISH. 505 



the waters in which they live, is similar to that 

 of man, and air-breathing creatures generally, 

 upon the atmosphere ; the fluid becomes vitiated, 

 and a necessity is created for its renewal. This 

 process, as in the terrestrial world, is, as we 

 have seen, discharged by plants. There can 

 be little doubt also, that the mere effect of 

 constant agitation, as before noticed in the 

 phenomena of waves, causes the solution of a 

 large portion of oxygen ; for it is found that 

 sea- water contains also nitrogen in solution, 

 which has unquestionably been obtained from 

 the agitation of water and air together. The 

 oxygen of the rain which falls on the sea is 

 likewise an important addition to its contents. 

 We may, in fact, see in the grand circle of the 

 evaporation of water from the sea, of its conden- 

 sation in largest quantity over land, and of its 

 return by innumerable channels to the bosom 

 of its broad parent again, a beautiful system 

 for providing for the due oxygenization of the 

 ocean waters. For no method of dissolving 

 atmospheric oxygen could be devised more 

 complete than this. Rapids and cataracts effect 

 a similar object. Could the chemist draw from 

 the foaming pit of water into which the water- 

 fall at the other end of this valley leaped, a 

 sufficient amount of water to submit to analysis 

 for its gases, and were he to contrast it with a 



